In general, the otter board is used for net-mouth expanding of trawl net in the fishing, and has a variety of constructions such as monoplane or biplane structures, but its net-mouth expanding force must be adjusted according to the sort and purpose of fishing.
Heretofore, it has been usual for this purpose to change the attack angle of otter board with respect to the towing direction or to vary the surface area of main body of otter board.
However, the change of attack angle of otter board has a tendency of deteriorating the stability of net-mouth expanding force, so that this means has hardly been used. On the other hand, the variation of surface area of otter board necessitates to equip constantly the fishing boat with many sorts of otter board having different surface areas. This means a high equipment costs, and long time periods as well as dangerous operations for the exchange of otter boards. More specifically, Japanese Patent Laid Open No. 99881/1974 and Japanese Utility Model Laid Open No. 106458/1985 and the like disclose biplane otter boards, wherein the otter board members are fixed with each other and occupy large spaces respectively, so that the disadvantages described above are serious.
Therefore, an alternative means is disclosed as described in Japanese Patent publication No. 16858/1976 and Japanese patent Publication No. 42711/1977.
In there methods, drain through-holes or slit through-holes are formed through otter board having relatively a large thickness, wherein water flow rate through said through-holes are adjusted by regulating the opening thereof by means of closing valves or otter flaps, thereby to vary the net-mouth expanding force of otter board.
However, since openings of through-holes formed in the inner surface of respective otter boards are fixed and said closing valves or otter flaps are provided at the bottom of respective through-holes, the net-mouth expanding force of otter board can be varied relatively in a small extent, so a large variation thereof can not be easily obtained.
For eliminating the above disadvantages of heretofore known otter board, it is an object of the invention to create a novel otter board having a simple construction which can adjust in a large extent the net-mouth expanding force of otter board thereby to expand the trawl net precisely according to the sort of fishing and which can be used in the whole depth of sea for bottom, lower, middle and upper layer trawlings.